I want a pop up to display when customer puts products from category A into the cart.

I want a pop up to display when customer puts products from category A into the cart.
When they spend £100 on category A they get 15% discount from category B.
(discount is already configured)
Pop up should display products from category B.

How do I set this up – the upsells page only seems to have products as a trigger, not categories?